Chemical analysis of a test element often involves the use of UV/visible radiation from 300 nm to 700 nm, to determine quantitatively the composition of liquids like water, foodstuffs or biological fluids like serum. Reflectance or transmission density, or fluorescence measurements of the test element can be used to calculate chemical concentrations thereby providing a more complete picture of a patients condition.
In dry chemistry blood analysis (as used herein, "dry chemistry" refers to tests wherein there are no liquid reagents stored for use, such tests being possible by test elements of the type described in U.S. Pat. No. 3,992,158, issued Nov. 16, 1976), a sample of blood serum, is added to a test element containing chemical reagents. After sufficient incubation time, a color change or fluorescence is detected by a radiometer. The following is a typical example of a colorimetric test for albumin: ##STR1## where the BCG-Albumin complex, formed upon the interaction of albumin with the indicator bromocresol green (BCG), absorbs radiation and the absorption maximum is at 630 nm. The degree of radiation absorption can be monitored as a measure of the albumin concentration.
These tests can be performed, as described in U.S. Pat. No. 3,992,158, issued Nov. 16, 1976, in clinical chemistry slides. The test element shown in FIG. 1 includes a multilayered analytical element coated on a clear polymeric support 12 held in a slide mount 11. The top layer 18 is an isotropically porous spreading layer which evenly distributes the fluid to be analyzed into the underlaying reagent layers 20, that may perform multiple functions in an integrated manner.
The trend in this industry is toward smaller more compact instruments such that their use could be extended to smaller institutions and individual practices.
The prior art includes several types of light sources that can be used for making measurements in the 300 nm to 700 nm range such as tungsten halogen and pulsed xenon lamps. One of the problem associated with these radiation sources is their relatively low stability, short life, size, and lack of compactness and ruggedness. Although there is a high ratio of usable radiant energy with respect to power and heat generated, the measurement of that power is limited to a short time. High voltages and sudden surges of high current generate electrical noise that often interferes with other electronic subsystems. Furthermore tungsten halogen lamps have relatively short lives, generate excessive heat, and have a low ratio of usable light to power in the UV-blue region of the spectrum. In addition the output power or intensity decreases with time and with filament life.
These lamps, in general, are large and cumbersome and require frequent adjustments and replacement because of burned out filaments. In addition these lamps preclude efforts to miniaturize such equipment and also pose a potential danger to operators and to the equipment itself because of the heat generated during their operation. The wavelengths produced may vary as a function of time with the aging of the lamps, and because the process involves Ar or halogen gases that have to be "ignited" in order to produce radiation, it is not possible to modulate these lamps at rates faster than what a mechanical shutter would provide.
FIG. 2 shows a schematic of a prior art system. As shown in FIG. 2, light produced by a lamp 47 is passed through an upconversion device 48, an infrared filter 50 and is reflected off of a second (cold) mirror 52 which absorbs infrared radiation through an aperture arrangement 54 on through a series of lenses 56 where a light is focused onto a sample deposited on a slide 58. The sample includes a liquid that has an active biological fluid and a chemically interactive material. See the discussion in the Background of the Invention. Light which is reflected off of the slide through the biological fluid passes through a collimated lens assembly 60 and then a relay lens assembly 62 where it is focused onto a photodetector 59. Intermediate between the collimated lens assembly 60 and the relay lens assembly 62 there is provided a filter wheel 64 through which appropriate filters can be placed into the optical path of the light between the collimated lens assembly 60 and the relay lens assembly 62. The filters are used to select the spectrum of light that will interact with the chemically active material found in the test element. The sample containing the biological fluid also contains a chemically active material, that can be activated in the presence of such material to produce a quantitative and detectable change which can be the generation or destruction of coloration or fluorescence. Besides the above discussed lighting problems, one of the drawbacks of such an approach lies in the fact that it requires a large number of optical elements to guide the "interrogating radiation" into the slide. Furthermore, it requires to bring the UV radiation from an external source along with the special handling requirements of such UV radiation.